The present invention pertains to steel strap. More particularly, the present invention pertains to a composition of a cold rolled full hard steel strap and a method of making strap for use in strapping machines for providing a tensioned loop about packaged articles.
Articles are often packaged in a bundle, on a pallet or in a crate for shipping, storage and merchandising. Many times, such bundled articles are secured with a steel or polymer strap applied in a tensioned loop by an automatic or manually operated strapping machine. Some applications, and in particular those applications in which the strap secures a package having substantial weight, such as a stack of bricks, lumber and the like, require the use of a steel strap which has high tensile strength and is less susceptible to deterioration by abrasion than polymer and existing metal strap. Further, although certain existing steel strap is readily applicable to heavy packaged articles having cylindrical shapes and otherwise smooth or obtuse surfaces, there are limitations on the extent to which it can be formed under tension over and around sharp edges and corners of a package.
More specifically, packages having sharp edges or corners with a small radius of curvature, for example a 90° corner, pose a problem for existing steel strap because the strap is subject to tremendous stress and strain as the strap tension is increased to an extent necessary to secure the packaged article. This stress and strain frequently causes the strap to fracture proximate to the edge or corner of the packaged article. In particular, the relatively low ductility of non-heat treated strap contributes to the failure of strap used in this application. Moreover, the problem is exacerbated when the strap is applied and tensioned with an automatic strapping machine that generates a high tension in a short time interval during a rapid strap application process.
Many practices have been developed to reduce strap failure, such as reducing the tension applied to the strap or placing a shield between the articles to be bundled and the strap. However, reducing strap tension may result in insecurely packaged articles and the use of shields requires an additional step that is time consuming and can be labor intensive, thus increasing costs. As such, these practices are not practical for long term, cost efficient strapping operations.
Crystalline metals, such as steel, are comprised of lattice structures that include imperfections, or “dislocations”. Three types of such imperfections, well known in the prior art, are vacancies, interstitial atoms and substitutional atoms (collectively known as “point defects”). In most conventional steel products, including steel strap, such imperfections traditionally have been deemed undesirable because, while the existence of such imperfections generally helps increase strength in cold rolling applications, the imperfections also detrimentally affect the steel's formability and ductility in such applications, and result in the need for subsequent heat treatment after cold rolling to restore formability and ductility.
Strain hardening, such as cold rolling during cold reduction, is one of the most commonly used means of strengthening steel and is well known in the prior art. In traditional cold rolling of steel products, cold reduction is done primarily to achieve a thinner gauge steel than can be otherwise obtained directly from hot mill rolling. However, cold reduction also increases imperfections as a result of plastic deformation and yields a very brittle and unformable steel sheet, which typically must be subsequently annealed, or “heat treated,” to remove the hardening caused by the imperfections created, and deformed, by the cold reduction. Thus, the prior art has focused on improving the formability of steel by reducing such imperfections rather than by intentionally increasing them.
Typical standard steel strapping (non-heavy duty strapping) is manufactured by cold reduction with no subsequent annealing (full hard). In the absence of the annealing process, desirable physical strapping properties, such as tensile strength and formability, are developed through other means, such as the chemical composition of the steel, the finishing and coiling temperatures, and the amount of cold reduction.
With respect to chemical composition, iron-based materials suitable for steel strap generally include carbon which is added to the steel to increase the tensile strength of the strap. The addition of carbon, however, creates interstitial imperfections and tends to increase embrittlement, which decreases formability and, accordingly, the ability of steel strap to be formed over and around corners without fracturing.
The prior art also teaches the addition to, or removal of, other elements in a steel's composition to impart various desired physical properties. However, the combination and amount of such elements also controls the types of point defects that are formed, and can enhance the desired physical properties, such as tensile strength, through solution hardening. For example, aluminum and silicon, generally added to remove excess oxygen and nitrogen, both create substitutional imperfections, which help increase strength.
Substitutional imperfections also are formed when alloying various elements with steel. Manganese and nickel, typically added to increase a steel's tensile strength (and, in the case of manganese, to react with sulfur), create substitutional imperfections by replacing iron atoms in the steel crystalline lattice structure. Chromium, which is added to increase hardness and melting temperature, also creates substitutional imperfections. Molybdenum, added to help harden a steel, creates substitutional imperfections. Copper, also generally added to increase hardness, creates substitutional imperfections. Atoms of the foregoing elements in the steel crystalline lattice structure distort the steel crystals, impeding slip and increasing the yield strength of the steel.
Finally, while sulfur, nitrogen, and phosphorus tend to make steel more brittle, and these elements generally are removed or minimized, their presence in controlled amounts also creates substitutional imperfections that may increase strength.
Similarly, control of the finishing and coiling temperatures during hot mill rolling is known in the prior art as an important factor in determining the tensile strength of a steel. Also known in the prior is that the reduction of steel by cold working increases the steel's tensile strength, as discussed above. As such, reduction of steel by cold working allows the carbon content can be reduced while still maintaining a fixed tensile strength. However, the reduction of steel by cold working also increases steel embrittlement and decreases steel formability. In applications where steel formability is important, therefore, reduction by cold working has been performed to a limited extent to avoid embrittlement and the consequent loss in steel formability, and often is complemented by heat treatment (annealing) to restore formability. This adds time and cost to the steel production process.
Accordingly, there is a need for a high tensile strength steel material suitable for use in making steel strap. Desirably, such a strap material exhibits a high tensile strength without the undesirable properties of reduced ductility and increased brittleness as commonly occur in association with the manufacture prior art steel strap materials. More desirably, such a strap is manufactured by cold reduction with no subsequent annealing. Most desirably, such a strap material provides increased tensile strength as a result of intentionally created imperfections in the steel crystalline lattice structure.